As a transmission assembly of the type set forth above, there is known from U.S. Pat. No. 4,579,183 a transmission assembly in which almost all of the space within a transmission casing is utilized effectively so as to reduce the size of the casing.
In the transmission assembly disclosed in this U.S. Patent, a hydrostatic transmission is mounted on the front of a transmission casing at an upper side of a mid-PTO shaft which extends forwardly from the casing. Motor shaft of a hydraulic motor which is located at a lower side in the hydrostatic transmission is used as an input shaft for transmitting the wheel-driving power into the transmission casing, whereas pump shaft of a hydraulic pump which is located at an upper side in the hydrostatic transmission is projected rearwardly into the transmission casing and is used as an input shaft for transmitting the PTO shaft-driving power into the casing.
Within the transmission casing, a wheel drive assembly is arranged at a lower half in the casing. This wheel drive assembly comprises a change speed gearing disposed between a drive shaft, which is arranged at a front portion within the transmission casing and at a level lower than that of the motor shaft and is drivenly connected to the motor shaft via meshing gears, and a speed-change shaft which is arranged at a front portion within the transmission casing and at a level lower than that of the drive shaft. The wheel drive assembly further comprises a differential gearing which is arranged behind the change speed gearing and is connected drivenly to the speed-change shaft. Left and right output shafts of the differential gearing are disposed co-axially with left and right rear wheel axles and are connected drivingly to such axles through left and right speed-reduction planet gearings, respectively. In addition, an output shaft for front wheel-driving power is also arranged at a front portion within the transmission casing and is connected drivenly to the speed-change shaft. Such output shaft is disposed such that it extends forwardly from the transmission casing at a location spaced laterally from a mid-PTO shaft which also extends forwardly from the casing.
On the other hand, a PTO shaft drive assembly is arranged at an upper half within the transmission casing and includes, as a drive shaft, a transmission shaft which is arranged coaxially with and coupled to the pump shaft referred to before. Below this transmission shaft are arranged a clutch shaft and intermediate shaft which are spaced from each other in a direction across the transmission casing. On the clutch shaft are mounted a fluid-actuated PTO-clutch of the multidisk type for connecting between the transmission shaft and clutch shaft, a brake for stopping an inertial rotation of the clutch shaft quickly when the PTO-clutch is disengaged, and a PTO-selecting clutch for transmitting rotation of the clutch shaft selectively to a rear-PTO shaft and to the mid-PTO shaft. The rear-PTO shaft which extends rearwardly from the transmission casing at a mid level of the casing is driven to rotate by one of a pair of clutch gears of the PTO-selecting clutch via the intermediate shaft, whereas the mid-PTO shaft is driven to rotate by the other clutch gear via a gear train which is disposed at one side of the front portion within the transmission casing.
In the transmission assembly disclosed in U.S. Pat. No. 4,579,183 in which the pump shaft and motor shaft of a hydrostatic transmission are used respectively for input shafts for transmitting the PTO shaft-driving power and wheel-driving power into a transmission casing, the PTO-shaft drive assembly and wheel drive assembly are concentratedly arranged respectively at an upper half and at a lower half within the transmission casing in correspondence with a higher level of the pump shaft and a lower level of the motor shaft in the hydrostatic transmission so that the transmission mechanism within the transmission casing is arranged in a compact fashion so as to reduce the size of the casing. However, such transmission assembly still involves some problems which will be detailed hereinafter.
With respect to the wheel drive assembly arranged at a lower half within the transmission casing, the drive shaft and speed-change shaft set forth before are arranged below the motor shaft successively in that order at a central portion of the lateral width of the transmission casing in correspondence with the arrangement of the motor shaft that it is projected into the transmission casing at such central portion of the width of the casing. Such arrangement of shafts requires a relatively large vertical space for the wheel drive assembly. For reducing such vertical space, it is considered to arrange the drive shaft and speed-change shaft in a laterally spaced relationship so that the speed-change shaft is disposed at substantially the level of the drive shaft. It is, however, difficult to adopt such arrangement of shafts due to a restriction given by the aforestated gear train which connects between the PTO-selecting clutch and mid-PTO shaft. Further, left and right output shafts of the differential gearing which is driven by the speed-change shaft are located at a low level corresponding to the level of the speed-change shaft so that, in a case where such left and right output shafts are connected drivingly to left and right rear wheel axles through left and right speed-reduction gearing each having a pair of meshing spur gears, it is required for securing a vertical space for such meshing spur gears to further enlarge or extend the transmission casing downwardly. From this, it is obliged for avoiding an undue hightening of the transmission casing to arrange the left and right rear wheel axles co-axially with the left and right output shafts of the differential gearing and to connect between such output shafts and axles through left and right final reduction means each consisting of a relatively expensive planet gearing, as is the case in the transmission assembly disclosed in U.S. Pat. No. 4,579,183.
With respect to the PTO-shaft drive assembly, although the aforestated clutch shaft mounting thereon the PTO-clutch, brake and PTO-selecting clutch permits an easy assembling of the clutch mechanism into the transmission casing, such clutch shaft and another shafts associated thereto are lengthy so that axial length of the transmission casing is relatively large.